Differentiation of arrays of cells in the intact organism is accomplished by sequential display of specific molecules capable of transmitting information about their surroundings through intercellular intermediates to effect a change in the cell's genetic program. To approach the role of intercellular contact in determining differentiation, we propose to develop an in vitro system capable of mimicking cell-cell interactions, specifically between prothymocytes and thymic stroma cells. In this system, intimate cellular contact between the thymic stroma and the prothymocyte probably plays a major role in the initiation of differentiation of the prothymocyte into the major T-cell lineages. To test this hypothesis we propose to derive in vitro functional correlates of each cell type and to this end we will: A. Characterize the morphologically distinct stromal cell types we have clonally derived in vitro from the hyperplastic thymus of an SV40 transgenic mouse and determine the interactions that lead to proliferation and differentiation of the prothymocyte. B. Derive in vitro stable populations of replicating prothymocytes and determine if they are capable of differentiation by means of interaction with the thymic stromal cell lines.